The preparation of the pulp from cellulose until the manufacture of paper comprises several steps which are summarized, for example, in PCT/US 90/02823 patent application, as follows:
Wood is comprised of two main components--a fibrous carbohydrate, i.e., cellulosic portion, and a non-fibrous component. The polymeric chains forming the fibrous cellulose portion of the wood are aligned with one another and form strong associated bonds with adjacent chains. The non-fibrous portion of the wood comprises a three-dimensional polymeric material formed primarily of phenylpropane units, known as lignin. Part of the lignin is between the cellulosic fibers, bonding them into a solid mass, although a substantial portion of the lignin is also distributed within the fibers themselves.
For use in paper-making processes, wood must first be reduced to pulp. Pulp may be defined as wood fibers capable of being slurried or suspended and then deposited upon a screen to form a sheet, i.e., of paper. The methods employed to accomplish the pulping step usually involve either physical or chemical treatment of the wood, or a combination of these two treatments, to alter the wood's chemical form and to impart desired properties to the resultant product. There are thus two main types of pulping techniques, i.e., mechanical pulping and chemical pulping. In mechanical pulping, the wood is physically separated into individual fibers. In chemical pulping, the wood chips are digested with chemical solutions to solubilize a portion of the lignin and thus permit its removal. The commonly utilized chemical pulping processes are broadly classified as: (1) the soda process, (2) the sulfite process, and (3) the Kraft process, with the latter process being most commonly used.
In mechanical pulping, the wood fibers are separated from each other by mechanical action of grinders or rotating plates. The mechanical action may be accompanied by the application of certain chemicals to reduce the energy required or to bleach the fibers.
During all of these processes and also during bleaching and drying, the pulp moisture content is reduced to levels at which the mixture is combustible. This most commonly occurs in the final process called "Flash Drying". In mills where the production of paper is not integrated, the pulp is dried in the flash drier and bales of pulp are made to be further shipped to paper manufacture plants.
Furthermore, in order for certain bleaching reactions to occur, the pH of the slurry must be controlled to the range exceeding 9. This level is too high for processing into paper in subsequent processes and an acid treatment is required to lower the pH to the range of 7. Pulp stored at high pH levels are subject to darkening which is prevented by processes described in Canadian Patent No. 1,288,906 in which carbon dioxide is the preferred acid. The use of carbon dioxide at processing stages prior to the pulp dryer, in existing mills, is limited by the difficulty in dissolving the gas in the slurry because of high temperatures and short residence times. In new mills, this can be overcome by proper process equipment design.
In the usual process of manufacturing pulp, said pulp is received in the flash dryer as an aqueous solution having a 50-60% consistency and a pH in the range of 7-8 if sulfuric acid is injected in the aqueous solution. However, sulfuric acid is an environmental concern and the acidic effluents of the mill have to be treated and recycled. If no sulfuric acid is used at this stage, the pH of the pulp is about 11.
However at the output of the baler, the pulp which is now about 90% consistency has to meet the customer's pH specification which is usually about 8 or lower. Thus, there exists a need to decrease the pH of the pulp when it comes out of the baler without using o substantially using sulfuric acid upstream in one stage of the process.
In addition to the above problem and with no relationship with it, the baler is a location well identified in the process where a fire can happen, essentially because of the low moisture content and presence of dust. Sources of ignition may be static spark, overheated pulp, or mechanical friction.
As in any such area, it is usually provided for fire protection with systems like the sprinkler shower system using water or a carbon dioxide snow system using cylinders of carbon dioxide which might be sprayed over the pulp as snow to avoid fire propagation.
It is known, e.g. from Canadian Patent 1,288,906, to use carbon dioxide for the preparation of paper pulp comprising a single or multistage bleaching of the pulp comprising at least one single or last peroxide bleaching stage or treatment followed by treatment of the bleached pulp with an aqueous solution of carbon dioxide, i.e. carbonic acid. As carbonic acid does not destroy the bleaching agent residuals, these residuals continue their bleaching action and prevent bacteria proliferation responsible for the pulp darkening.
However, until now, nobody has even thought about or suggested to simultaneously solve the two independent problems disclosed hereabove by using the same means in order to both control the pH of the pulp in or at the output of the baler and to avoid fire ignition or propagation in the said baler, and thus permit the retrofit of the process to existing mills.